The host range of and host-cell susceptibility to the leukosis-sarcoma virus is dependent on specific interactions between genetically controlled properties of the viral envelope and the host-cell plasma membrane. The cell membrane "receptor" may act as an inherited alloantigen and be detectable by existing serological methods. In an effort to produce alloantibodies specific for receptors controlling susceptibility to the viral subgroups A,B, and C, chickens of known receptor genotype and compatible erythrocyte alloantigens at 11 other loci will be cross-transfused. The resulting antisera will be analyzed using lines of chickens fixed for different combinations of A,B, and C receptors and of known alloantigen constitution. Another primary objective is to investigate the substructure of the erythrocyte alloantigen-determining portion of the B histocompatibility complex. The significance of this investigation rests on the finding that certain B haplotypes either resist specific tumor formation or are very effective in regressing them. Currently, several recombinants within the B complex are under investigation for effects of substructure components.